Solution for forming thermal resisting polymers

ABSTRACT

A concentrated solution suitable for forming thermal resisting polymers having a high degree of polymerization comprising a mixture of a prepolymer solution prepared by the reaction of an organic diisocyanate or diamine and a molar excess of 1,2,3,4-butanetetracarboxylic acid or an anhydride of the acid, and a blocked polyisocyanate.

This is a continuation of application Ser. No. 347,761, filed Apr. 4,1973, now U.S. Pat. No. 3,896,089.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solution for forming thermalresisting polymers and more particularly it relates to a concentratedsolution suitable for forming a highly polymerized thermal resistingpolymers having an imide ring and, as the case may be, a hydantoin ringor an amide linkage in the main chain of the polymer by heating thesolution. The invention relates further to a process of preparing such asolution.

2. Description of the Prior Art

It is known that polyimides, polyamideimides, polyimidazoles, orcopolymers thereof have high thermal resistance, high chemicalresistance, and other quite excellent properties and, in particular,these polymers or copolymers are very useful as materials for preparingwire coats, films, laminates, coating materials, adhesives, impregnationvarnishes, etc., to be used at high temperatures.

These thermal resisting polymers have generally been prepared byreacting starting materials, e.g., a tetracarboxylic acid dianhydrideand a diamine in case of preparing polyimide, in an organic polarsolvent such as N-methyl-2-pyrrolidone, N,N-dimethylformamide,N,N-dimethylacetamide, etc., to provide a polymer precursor and thensubjecting the polymer precursor to a treatment such as a heat treatmentto provide the desired thermal resisting polymer. Since the thermalresisting polymer thus obtained has generally been converted into aninfusible and insoluble material, the final polymer is poor inprocessability. Accordingly, processing operations are usually conductedon the aforesaid liquid polymer precursor having a sufficiently highmolecular weight, and in this case it is desired, from the aspects ofworkability and economy, that the solution of the polymer precursor beof high concentration and low viscosity.

However, in order to obtain a processed final polymer having goodproperties, the polymer precursor must have a sufficiently highmolecular weight. On the other hand, the solvent used for thepreparation of the polymer precursor is, in general, expensive, andfurther there is a limit on the solubility of the polymer precursor inthe solvent. Therefore, if a polymer precursor having a sufficientlyhigh molecular weight is desired, the solution thereof inevitablybecomes highly viscous, which results in reducing the working property.If it is desired to reduce the viscosity of the solution of the polymerprecursor, a large amount of the solvent must be used, which results inmaking the production uneconomical and reduces the concentration of thepolymer precursor.

Thus, in order to prepare a solution of the polymer precursor suitablefor processing, a large amount of organic polar solvent is required andsince such an organic polar solvent is expensive, the solution of thepolymer precursor becomes expensive. Therefore, in spite of theirexcellent properties, conventional thermal resisting polymers have notyet been used in wide fields but have been used only for specificpurposes.

The inventors have investigated developing solutions for forming thermalresisting polymers excellent in working property and economical aspectswhich are not accompanied by the aforesaid faults, and as a resultthereof, the inventors have discovered that a highly concentrated lowviscosity solution for forming thermal resisting polymers can beobtained in an inexpensive solvent without using an expensive organicsolvent or with a greatly reduced amount of expensive organic polarsolvent.

SUMMARY OF THE INVENTION

That is, according to the present invention there is provided asolution, particularly a concentrated solution for forming thermalresisting polymers, comprising a mixture of a prepolymer solution and ablocked, stable polyisocyanate compound in an approximatelystoichiometric equivalent amount to the amount of the prepolymer, theabove prepolymer having been prepared by reacting an organicdiisocyanate or diamine and a molar excess amount of1,2,3,4-butanetetracarboxylic acid or an anhydride thereof and having animide group in the main chain of the molecule and an acid group at bothterminals of the molecule.

By subjecting the aforesaid concentrated solution of the prepolymer thusprepared (after processing properly according to the use) to aclose-ring polycondensation by heat treating it while evaporating offthe solvent of the solution, the prepolymer solution can be convertedinto a polymer having sufficient toughness and high thermal resistance.

DETAILED DESCRIPTION OF THE INVENTION

The prepolymer solution used in this invention is prepared by reactingunder heating an organic diisocyanate or an organic primary diamine anda molar excess of 1,2,3,4-butanetetracarboxylic acid or an anhydridethereof in an organic acidic solvent such as a phenol, a cresol, axylenol, etc., or as the case may be, an organic polar solvent such asN,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone,etc.

The aforesaid solvent may be also used together with an organicnon-polar solvent such as toluene, xylene, benzene, solvent naphtha,etc.

The amount of the 1,2,3,4-butanetetracarboxylic acid or the anhydridethereof to be used in the above reaction is 1.1 to 2.2 mols, preferably1.3 to 2.0 mols, per mol of the organic diisocyanate or the organicprimary diamine. The concentration of the reaction system is preferably30 to 95% by weight, in particular, 40 to 65% by weight for increasingthe reaction rate. Furthermore, when the aforesaid reaction is conductedat temperatures above 30° C, preferably at temperatures of 130° to 200°C, better results such as high reaction rate and less occurence of sidereactions can be obtained.

Now, the production of the prepolymer solution by the reaction of1,2,3,4-butanetetracarboxylic acid (hereinafter, the acid is called"BTC") or an anhydride thereof (hereinafter, the anhydride is called"BTCA") and an organic diisocyanate or an organic primary diamine willbe explained practically by the following examples.

1. Reaction of BTC-organic primary diamine system:

This reaction is a dehydration reaction and since the dehydrationreaction occurs abruptly at reaction temperatures of 130° to 140° C,water formed in the dehydration reaction is removed from the reactionsystem. As the reaction progresses, the color of the solution changesgradually to black-brown. Furthermore, when the reaction temperature isfurther raised and the reaction is continued at 190° to 200° C, thedistillation out of water ceases after about 20 to 30 minutes. The endof the reaction, that is, the formation of the imide group-containingtetracarboxylic acid prepolymer, is confirmed by estimating the contentof acid by an alkali method. The reaction is completed when the reactionis carried out for about 2 to 3 hours after raising the temperature ofthe reaction system to 190° to 200° C, and the reaction does not thenproceed even if the heating of the system is further continued.

Also, if it is desired to further decrease the viscosity of the solutionin the reaction, the polymerization may be conducted in the presence ofa monoalcohol, a dialcohol, or a trialcohol.

This reaction does not stop in the form of amide acid as an imidationreaction proceeds at almost the same temperature as the amidation, andthe imide group-containing tetracarboxylic acid prepolymer has excellentsolubility in such inexpensive solvent as cresols, xylenols, etc. Also,since the carboxyl group of BTC is active, the amidation reactionproceeds at a higher reaction rate than the rate of forming an amidegroup from a carboxyl group bonded to an aromatic ring and an aminogroup generally conducted, and then the imidation reaction proceeds atthe same temperature.

The above is explained for the case of using BTC, but an imidegroup-containing tetracarboxylic acid prepolymer having a similar acidcontent can be obtained by using BTCA.

2. Reaction of a BTC-organic diisocyanate system:

This reaction is a carbon dioxide removal reaction and dehydrationreaction and since the formation of water and carbon dioxide occursabruptly at reaction temperatures of 130° to 140° C, these side reactionproducts are removed from the reaction system. As the reactionprogresses, the color of the solution changes gradually to black-brown.When the reaction temperature is further raised and the reaction iscontinued at 190° to 200° C, the distillation out of water ceases afterabout 20 to 30 minutes.

The reaction is finished about 2 to 3 hours after the increase of thereaction temperature to 190° to 200° C, and the reaction does notproceed even if the heating is continued. It is preferable to use as thesolvent for the reaction an organic acidic solvent such as a phenol, acresol, a xylenol, and the like.

Furthermore, it has been discovered that in the reaction, thepolycondensation of the BTC and the organic diisocyanate occurs veryeasily to form an imide group-containing tetracarboxylic acidprepolymer, the structure of which is the same as that of the case ofreacting BTCA and the organic diisocyanate.

It is well known that an isocyanate group is quite a reactive functionalgroup and is caused to react with compounds having active hydrogen toprovide various compounds. In particular, it is feared that various sidereactions occur caused by the reaction of the isocyanate group andwater. However, in the reaction of BTC and the organic diisocyanate, theaforesaid side reactions scarcely occur and a prepolymer the same as theimide group-containing tetracarboxylic acid prepolymer obtained by theaforesaid reaction (1) is obtained.

That is, it has been believed that in the reaction of BTC and an organicdiisocyanate an isocyanate group is first caused to react with acarboxyl group to cause dioxide removal reaction and to form an amideacid, and then a dehydration reaction occurs at almost the sametemperature to easily cause the imidation, and thus it is an astonishingphenomenon that in spite of the occurence of the dehydration reactionthe isocyanate group actively acts to cause the imidation reactionwithout side reactions, and the imide group-containing tetracarboxylicacid prepolymer having a theoretical acid content is obtained.

As the blocked polyisocyanate compounds to be blended with the aforesaidprepolymer for providing the solution of this invention, there areillustrated the blocked compounds obtained by reacting a polyisocyanateand a compound represented by the formula

    ROH

wherein R represents a monovalent aromatic group, a monovalent alicyclicgroup or a monovalent aliphatic group.

Among these blocked compounds, the blocked compound of a polyisocyanatehaving at least one of an imide group, a hydantoin group and an amidegroup in the main chain of the molecule gives particularly preferableresults for obtaining the concentrated solution for forming polymershaving good thermal resistance, good mechanical properties and goodelectric properties. However, the blocked compounds of such diiso- ordithioisocyanates as p-phenylenediisocyanate, 2,4-tolylenediisocyanate,1,5-naphthylenediisocyanate, 4,4'-diphenylmethane diisocyanate,4,4'-diphenylether diisocyanate, hexamethylene diisocyanate,cyclohexane-1,4-diisocyanate, p-phenylene dithioisocyanate,4,4'-diphenylmethane dithioisocyanate, etc., as well as the blockedcompounds of polyisocyanate having a comparatively low molecular weightsuch as Desmodur AP-Staple, Desmodur CT-Staple, (trade names, made byFarbenfabriken Bayer Aktiengesellshaft), and the like can be also usedin this invention.

In addition, examples of the compounds having the formula ROH used inthe above reaction include phenols, cresols, xylenols, and alcohols.

Furthermore, examples of the aforesaid blocked compound ofpolyisocyanate having at least one of an imide group, a hydantoin groupand an amide group are as follows:

A. A blocked compound of an imide group-containing polyisocyanateprepared by dissolving BTC or BTCA and a molar excess amount of anorganic diisocyanate in a solvent such as a phenol, a cresol, a xylenol,or a mixture thereof and reacting them by heating.

The amount of the organic diisocyanate used in this reaction is 1.1 to2.2 mols, preferably 1.3 to 2.0 mols, per mol of BTC or BTCA. When theabove components are reacted by heating in the aforesaid solvent, carbondioxide is split off and a dehydration reaction occurs at 130° to 140°C, but the side reaction of the water formed and the isocyanate grouphardly occurs. This is considered to be based on the fact that when thereaction temperature of the reaction system reaches near 130° C (atwhich temperature the dehydration reaction occurs), the isocyanate groupis stabilized by being blocked with the phenol, cresol, or xylenol usedas the solvent for the reaction.

When the reaction is further raised and the reaction is conducted at150° to 200° C, the isocyanate group forms an imide group by reactionwith BTC or BTCA to provide the imide group-containing polyisocyanate,and when the temperature is reduced to below 150° C, the terminalisocyanate groups are caused to react with the solvent to form theblocked compound.

It is preferable, for increasing the reaction rate, that theconcentration of the raw materials to be reacted be 10 to 90% by weight,preferably 30 to 60% by weight.

The concentrated solution of this invention prepared by mixing theblocked compound of the imide group-containing polyisocyanate preparedabove and the above imide group-containing tetracarboxylic acidprepolymer solution is effective, when used as varnish for electricwires, for providing thermal resisting coated wires having excellentFreon resistance.

B. A blocked compound of an amide and imide group-containingpolyisocyanate having isocyanate groups at both terminals of themolecule prepared by reacting the organic tribasic acid anhydriderepresented by the general formula ##STR1## wherein R₁ represents atrivalent organic group and a molar excess amount of an organicdiisocyanate.

As the organic tribasic acid anhydride to be used in the above reaction,there are illustrated trimellitic anhydride,4-carboxydiphenylmethane-3',4'-dicarboxylic acid anhydride,3-carboxydiphenylmethane-3',4'-dicarboxylic acid anhydride,4-carboxydiphenylketone-3',4'-dicarboxylic acid anhydride, and the like.Also, the aforesaid organic tribasic acid anhydrides may be partiallysubstituted by an organic dibasic acid such as terephthalic acid andisophthalic acid and in such case the above-mentioned organic dibasicacid acts as a component effective for forming an amide group.

The amount of the organic disocyanate used in this invention is 1.1 to4.0 mols, preferably 1.2 to 2.0 mols per mol of the organic tribasicacid anhydride. When they are reacted in an organic polar solvent at 50°to 210° C, preferably 80° to 130° C, the amide and imidegroup-containing polyisocyanate is prepared.

It is preferable that the concentration of the raw materials to bereacted be high, particularly 50 to 90% by weight.

This reaction proceeds gradually while generating carbon dioxide andwith the progress of the reaction, the color of the reaction systemchanges to black-brown. The end of the reaction can be determined byestimating the isocyanate group by an n-butylamine method. When theisocyanate content reaches the theoretical amount, the product isblocked by a phenol, a cresol, a xylenol, or an alcohol according to aknown manner, whereby the blocked compound of the amide and imidegroup-containing polyisocyanate is obtained.

The concentrated solution of this invention prepared by mixing theblocked compound thus prepared and the aforesaid imide group-containingtetracarboxylic acid prepolymer is effective, when used as varnish forelectric wire, for giving thermal resisting coated wires havingexcellent abrasion resistance and Freon resistance.

C. A blocked compound of a polyisocyanate having a hydantoin ring, apart of which may be, as the case may be, hydantoic acid or a loweralkyl ester thereof, in the main chain of the molecule, and havingisocyanate groups at both terminals of the molecule prepared by reactingthe glycine derivative represented by the general formula

    [R.sub.1 OOC(R.sub.2).sub.2 CHN].sub.2 R.sub.3

wherein R₁ and R₂, which may be the same or difference, each representsa hydrogen atom or a lower alkyl group and R₃ represents a divalentorganic group and a molar excess amount of an organic diisocyanate.

As the glycine derivatives used in the above reaction, aromatic ones arepreferable but aliphatic and alicyclic ones may also be used. Examplesof such glycine derivatives are p-phenylenebis (iminoacetic acid),m-phenylenebis(iminoacetic acid), 4,4'-diphenylenebis(iminoacetic acid),4,4'-diphenylbis(iminoacetic acid), 4,4'-diphenyletherbis(iminoaceticacid), 4,4'-diphenylmethanebis(iminoacetic acid),4,4'-benzophenonebis(iminoacetic acid),4,4'-diphenylsulfonebis(iminoacetic acid), p-cyclohexyenebis(iminoaceticacid), m-cyclohexylenebis(iminoacetic acid),hexamethylenebis(iminoacetic acid), the lower alkyl esters thereof, andisomers thereof.

The preferred amount of the organic diisocyanate used in this reactionis 1.1 to 4.0 mols, more preferably, 1.2 to 2.0 mols per mol of theaforesaid glycine derivative, and by reacting them in an organic acidicsolvent such as a cresol, a xylenol, etc., at temperatures of 30° to210° C, the polyisocyanate having a hydantoin and, as the case may be, afunctional group having a hydantoin ring forming faculty in the mainchain of the molecule and further having isocyanate groups at bothterminals of the molecule is prepared.

It is desirable that the concentration of the raw materials to bereacted be high, preferably 50 to 90% by weight.

When, after conducting the reaction for 20 to 60 minutes at 30° to 60°C, the temperature of the reaction system is gradually raised, thereaction proceeds while forming water in the case of using theiminoacetic acid, and generating alcohol in the case of using the alkylester of the iminoacetic acid at about 120° C. As the reactionprogresses, the color of the reaction system becomes black-brown. Whenthe reaction is further conducted at an elevated temperature of 190° to200° C for 3 to 4 hours, the polyisocyanate containing a theoreticalamount of isocyanate is obtained. After the reaction is over, thereaction product is cooled, whereby the isocyanate group of theaforesaid polyisocyanate is caused to react with the solvent thus usedto form the blocked compound.

The concentrated solution of this invention prepared by mixing theblocked compound thus obtained with the aforesaid imide group-containingtetracarboxylic acid prepolymer solution is effective, when used asvarnish for enameled wires, for giving thermal resisting wires excellentin flexibility and abrasion resistance.

The various kinds of blocked polyisocyanate compounds and the processesof preparing these blocked compounds were explained as above but inorder to obtain the concentrated solution of this invention, it ispreferred to mix the blocked compound as described above and the imidegroup-containing tetracarboxylic acid prepolymer in an approximatelystoichiometrically equivalent amount, but either of the blocked compoundor the prepolymer may be used in an excess amount of up to 20% more thanequivalent.

In addition, by the term "stoichiometrically equivalent amount" is meantthat two carboxyl groups of the aforesaid prepolymer are used to oneisocyanate group of the aforesaid blocked compound.

By mixing the aforesaid prepolymer and the blocked compound in astoichiometrically equivalent amount ratio, the solution of thisinvention capable of forming a thermal resisting polymer having a highmolecular weight and being excellent in electric properties, mechanicalproperties and thermal characteristics by heating the solution isobtained.

Furthermore, the imide group-containing tetracarboxylic acid prepolymerto be contained in the solution has a comparatively low molecular weightand has good solubility, while the blocked polyisocyanate compound alsohas similar properties. Accordingly, the viscosity of the resultantsolution can be markedly reduced.

Thus, the solution for forming thermal resisting polymers of thisinvention has such the feature that it can be used in a highlyconcentrated state. For example, in the case of use as a varnish forelectric wires, the non-volatiles content of the solution may be 20 to50% by weight and the viscosity of the solution may be 500 to 10,000c.p. (at 30° C), preferably 1,000 to 6,000 c.p. (at 30° C) and furtherin the case of use as a solution for forming films, the non-volatilescontent of the solution may be 30 to 60% by weight and the viscosity ofthe solution may be 30,000 to 150,000 c.p. (at 30° C), preferably 50,000to 120,000 c.p. (at 30° C). That is, since the solution of thisinvention can be used in a highly concentrated and comparatively lowviscosity state, the solution is very advantageous from working andeconomical aspects.

For preparing thermal resisting polymer films from such a solution ofthis invention, the solution must be heated to conduct thepolymerization reaction thereof and to remove the solvent. It ispreferable to add a polymerization accelerator to the solution of thisinvention. Examples of such a polymerization accelerator includeorganometal salts such as lead octylate, iron octylate, zinc octylate,lead naphthenate, iron naphthenate, zinc naphthenate, calciumnaphthenate, dibutyltin dilaurate, zirconiumacetyl acetonate,aluminumacetyl acetonate, and ironacetyl acetonate and tertiary aminessuch as dimethylbenzyl amine, pyridine, and triethylene diamine. Thepost hardening after removing the solvent for obtaining final films isgenerally conducted at temperatures of 150° to 500° C.

The solution of this invention can be stably stored for a long period oftime without being accompanied by deterioration. The solution of thisinvention can be used at any desired viscosity and concentrationaccording to various uses such as materials for forming films orcoatings as well as for impregnation varnishes, laminates, andadhesives.

Now, the production of the imide group-containing tetracarboxylic acidprepolymer used as one component for obtaining the solution of thisinvention for forming thermal resisting polymers will be illustrated inthe following Examples A - E and the production of the blocked compoundof polyisocyanate used as another component for obtaining the solutionof this invention will be illustrated in the following Examples F - H:

EXAMPLE A

In a 1 liter four-necked flask equipped with a Diens trap havingcondenser, a thermometer, and a stirrer were placed 234.2 g (1 mol) ofBTC, 158 g (0.8 mol) of 4,4'-diaminodiphenylmethane, and 392 g ofindustrial cresol and then the mixture was heated gradually withstirring.

The dehydration reaction occurred at 110° to 140° C and thus theazeotropic mixture of the water and a small amount of cresol weredistilled off. When the mixture was further reacted at a highertemperature of 190° C for 3 hours, 724 g of the concentrated solution ofa prepolymer having an acid content of 0.524 (theoretical value being0.523) was obtained. In addition, the term "acid content" is the valuerepresented by the ratio of the molecular weight of COOH to themolecular weight of the prepolymer.

Since the solution prepared above had a high viscosity, the solution wasdiluted by 386 g of industrial cresol to adjust the non-volatile contentto 30.8% by weight (after drying for 2 hours) at 200° C) and theviscosity of the solution to 2050 c.p. (at 30° C).

EXAMPLE B

In the same flask as in Example A were placed 234.2 g (1 mol) of BTC,125.0 g (0.5 mol) of 4,4'-diphenylmethane diisocyanate, and 323 g ofindustrial xylenol and then the mixture was heated gradually withstirring. The dehydration and carbon dioxide removal reaction occurredat 110° - 140° C and thus water and a small amount of xylenol weredistilled off. When the mixture was further reacted at an elevatedtemperature of 200° C for 2 hours and 30 minutes, 645 g of theconcentrated prepolymer solution having an acid content of 0.510(theoretical value being 0.501) was obtained. By diluting the solutionobtained with 70 g of industrial xylenol, the solution having anon-volatile content of 45.1% by weight and a viscosity of 82700 c.p.(at 30° C) was prepared.

EXAMPLE C

In the same flask as in Example A were placed 234.2 g (1 mol) of BTC,140.2 g (0.7 mol) of 4,4'-diaminodiphenyl ether, 100 g of industrialphenol, and 274 g of industrial xylenol and then the mixture was heatedgradually with stirring. The dehydration reaction occurred at 110° -140° C and the water was distilled off together with a small amount ofthe solvents. Thereafter, when the mixture was further reacted for 5hours at 180° C, 706 g of the concentrated solution of prepolymer havingan acid content of 0.502 (theoretical value being 0.515) was obtained.By diluting the solution with 105 g of industrial cresol, the solutionhaving a non-volatile content of 41.5% by weight and a viscosity of 8600c.p. (at 30° C) was obtained.

EXAMPLE D

In the same flask as in Example A were placed 234.2 g (1 mol), 180.2 g(0.9 mol) of 4,4'-diaminodiphenyl ether, and 414 g of industrial cresoland the mixture was heated gradually with stirring. The dehydrationreaction occurred at 110° - 140° C and thus the water was distilled offtogether with a small amount of the solvent. Then, when the mixture wasreacted for 3 hours at 190° C, 760 g of the concentrated solution havingan acid content of 0.440 (theoretical value being 0.458) was obtained.By diluting the solution with 760 g of industrial cresol, the solutionhaving a non-volatile content of 30.2% by weight and a viscosity of 820c.p. (at 30° C) was obtained.

EXAMPLE E

In the same flask as in Example A were placed 198 g (1 mol) of BTCA, 158g (0.8 mol) of 4,4'-diaminodiphenylmethane, and 356 g of industrialcresol and the mixture was heated to 80° - 90° C for 30 minutes withstirring. Then, when the temperature of the mixture was raised, thedehydration reaction occurred at 110° - 140° C, whereby the imidationreaction occurred and at the same time the terminal acid anhydridegroups of BTCA caused the ring-close. Thereafter, when the reactionproduct was maintained at 190° C for 3 hours, 682 g of the concentratedsolution of prepolymer having an acid content of 0.520 (theoreticalvalue being 0.515) was obtained.

By diluting the solution with 128 g of industrial cresol, the solutionhaving a non-volatile content of 39.8% by weight and a viscosity of12500 c.p. (at 30° C) was obtained.

EXAMPLE F

In the same flask as in Example A were placed 250 g (1 mol) of4,4'-diphenylmethane diisocyanate and 367 g of industrial cresol and themixture was heated to 150° - 165° C for 1 hour with stirring to blockthe isocyanate group by cresol. Then when 117 g (0.5 mol) of BTC wasadded to the mixture, the imidation reaction proceeded with thegeneration of water and carbon dioxide. During the reaction, the carbondioxide and water were distilled off together with a small amount ofcresol. Then, the reaction product was maintained at 150° - 165° C for 5hours and after adding thereto 45 g of industrial cresol, the mixturewas cooled. Thus, 655 g of the solution of the imide group-containingpolyisocyanate blocked compound having a NCO content of 0.241(theoretical value being 0.2407), a non-volatile content of 45.4% byweight, and a viscosity of 82400 c.p. (at 30° C) was obtained. Inaddition, the NCO content is a value represented by the ratio of themolecular weight of NCO to the molecular weight of the polyisocyanateblocked compound.

EXAMPLE G

In the same flask as in Example A were placed 250 g (1 mol) of4,4'-diphenylmethane diisocyanate, 153.6 g (0.8 mol) of trimellitic acidanhydride, 100 g of N-methyl-2-pyrrolidone, and 76 g of xylene and themixture was heated with stirring. A homogeneous solution was obtained atabout 70° C and when the temperature of the system was raised to 100° C,the reaction proceeded while generating vigorously carbon dioxide. Whenthe reaction was continued at the temperature, the color of the solutionchanged into black-brown after 2 hours and further the NCO contentreached the theoretical value (0.252) after 4 hours and 30 minutes.Thus, 683 g of industrial cresol was added to the reaction product andafter stirring it for 1 hour at 150° - 160° C to block the isocyanategroup by cresol, the product was cooled. Thus, the solution of thepolyisocyanate blocked compound having an imide group and an amide groupand further having a non-volatile content of 30.4 g by weight and aviscosity of 2150 c.p. (at 30° C) was obtained.

EXAMPLE H

In the same flask as in Example A were placed 250 g (1 mol) of4,4'-diphenylmethane diisocyanate, and 490 g of industrial xylenol andthen the former was dissolved in the solvent at 35° - 45° C. Then, afteradding to the solution 239.6 g (0.7 mol) of 4,4'-diphenylmethanebis(methyl iminoacetate), the reaction was conducted for 30 minutes at50° C. When the temperature was raised gradually, the demethanolreaction occurred at about 150° C and the viscosity of the systemincreased. The temperature was raised further while removing themethanol formed from the system and the reaction was further continuedfor 3 hours at 200° - 210° C. Then, after adding 173 g of industrialcresol, the product mixture was cooled, whereby the solution ofhydantoin ring-containing polyisocyanate blocked compound having the NCOcontent of 0.189 (theoretical value being 0.1889) was obtained. Thesolution had a non-volatile content of 39.7% by weight and a viscosityof 8750 c.p. (at 30° C).

Now, the preparation of the solution of this invention for forming heatresisting polymers will be practically explained by the followingexamples.

EXAMPLE 1

A mixture of 400 g of the imide group-containing tetracarboxylic acidprepolymer solution prepared in Example A and 400 g of the solution ofthe imide group-containing polyisocyanate blocked compound prepared inExample G was stirred well to provide the solution of this invention forforming thermal resisting polymers having a non-volatile content of29.8% by weight and viscosity of 2090 c.p. (at 30° C). When the solutionwas stored for 6 months at 30° C, the change of physical properties suchas viscosity and color was scarcely observed.

The solution prepared above was coated on the annealed copper wireshaving a diameter of 1.00 mm by means of a die and baked thereon attower temperature of 420° C and a rate of 6.0 m/min in a verticalresearch tower having a length of 3 m. The properties of the thusenameled wires are shown in Table 1.

EXAMPLE 2

A mixture of 306 g of the imide group-containing tetracarboxylic acidprepolymer prepared in Example E, 400 g of the solution of thepolyisocyanate blocked compound prepared in Example G, and 94 g ofindustrial cresol was stirred well to provide the solution of thisinvention having a non-volatile content of 30.6% by weight and aviscosity of 2230 c.p. (at 30° C). When the solution was stored for 6months at 30° C, no change in physical properties was observed.

The solution prepared above was used as varnish for enameled wires andcoated and baked on enameled wires as in Example 1. The properties ofthe enameled wires obtained are shown in Table 1.

EXAMPLE 3

A mixture of 400 g of the imide group-containing tetracarboxylic acidprepolymer prepared in Example B and 400 g of the solution of the imidegroup-containing polyisocyanate blocked compound prepared in Example Fwas stirred well to provide the solution having a non-volatile contentof 45.2% by weight and a viscosity of 82600 c.p. (at 30° C).

The solution prepared above was casted over a glass plate, dried andhardened under heating to 180° C for 30 minutes and then to 250° C for30 minutes, and then the film formed was stripped from the glass plateto provide a flexible film. The properties of the film thus obtainedwere as follows:

    ______________________________________                                        Thickness of the film  50 μ                                                Tear strength (ASTM 1004-61T)                                                                        370 g/mil                                              Tensile strength (ASTM 882-61T)                                                                      15.0 kg/mm.sup.2                                       Elongation (")         34%                                                    Tear resistance (JIS C-2318)                                                                         340 kg/mm                                              Volume resistivity     >10.sup.16                                             Dielectric constant    3.6                                                    Dielectric loss tangent                                                                              0.024                                                  ______________________________________                                    

EXAMPLE 4

A mixture of 400 g of the imide group-containing tetracarboxylic acidprepolymer prepared in Example C and 400 g of the solution of thehydantoin ring-containing polyisocyanate blocked compound prepared inExample H was stirred well to provide the solution having a non-volatilecontent of 41.0% by weight and a viscosity of 8670 c.p. (at 30° C).

The solution prepared above was casted over a glass plate in a thicknessof 50 microns and dried and hardened under heating to 180° C for 30minutes and then to 250° C for 30 minutes to provide a flat film havinghigh toughness and high scratch resistance.

Then, the solution prepared above was diluted by industrial cresol sothat the non-volatile content became 30% by weight and the resultantsolution was coated and baked on annealed copper wire having a diameterof 1.00 mm at temperature of 430° C and a rate of 6.5 m/min in avertical research tower having a length of 3 meters. The properties ofthe enameled wire thus obtained are also shown in Table 1.

EXAMPLE 5

To 850 g of the imide group-containing tetracarboxylic acid prepolymersolution prepared in Example D was added 26 g of Desmodur CT Staple(trade name, made by Farbenfabriken Bayer Aktiengesellshaft) and themixture was heated to 100° C with stirring to provide the solutionhaving a non-volatile content of 32.5% by weight and a viscosity of 1025c.p. (at 30° C).

A plain weave glass cloth having a thickness of 0.18 mm was impregnatedwith the solution prepared above so that the resin content became 50% byweight to the glass cloth and then the solution thus impregnated wasdried for 30 minutes at 180° C to remove the solvent.

Twelve sheets of the glass cloths thus processed were piled, heated for10 minutes between a press machine heated to 350° C under a lowpressure, and then pressed for 10 minutes under a pressure of 150kg/cm². The bending strength of the laminate thus obtained was 43 - 48kg/mm² at room temperature and 41 kg/mm² at 200° C and the waterabsorption factor thereof was 1.1%. Furthermore, the heating loss of thelaminate after storing for 10 days at 250° C was 1.0% by weight.

EXAMPLE 6

To 1400 g of the prepolymer solution prepared in the process of ExampleC were added 233 g of a blocked compound prepared by blockingdiphenylmethane diisocyanate with cresol, 250 g of cresol, and 0.02 g ofdibutyltin dilaurate and then the mixture was stirred at 100° - 120° Cto provide the solution having a non-volatile content of 43.2% by weightand a viscosity of 4230 c.p. (at 30° C).

The solution was coated and baked on an annealed copper wire having adiameter of 1.00 mm by means of a die under the same conditions as inExample 1. The properties of the enameled wire thus obtained are shownin Table 1.

EXAMPLE 7

To 1400 g of the prepolymer prepared by the process of Example C wereadded 199 g of the t-butanol blocked compound diphenylmethanediisocyanate, 190 g of cresol, and 0.6 g of lead octylate and themixture was stirred at 100° - 120° C to provide the solution having anon-volatile content of 43.5% by weight and a viscosity of 4330 c.p. (at30° C).

The solution was coated and baked on an annealed copper wire having adiameter of 1.00 mm by means of a die under the same conditions as inExample 1. The properties of the enameled wire thus obtained are shownin Table 1.

                                      Table 1                                     __________________________________________________________________________                           Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                   1    2    4    6    7                                  __________________________________________________________________________             Bare wire diameter (mm)                                                                     1.000                                                                              1.000                                                                              1.000                                                                              1.000                                                                              1.000                              Structure of                                                                           Enameled wire diameter(mm)                                                                  1.080                                                                              1.082                                                                              1.081                                                                              1.080                                                                              1.084                              wire     Film thickness (mm)                                                                         0.040                                                                              0.041                                                                              0.0405                                                                             0.040                                                                              0.042                              Appearance of                                                                          Naked eye evaluation                                                                        good good good good good                               wire                                                                          Windability                                                                            Good diameter 1 d  1 d  1 d  1 d  1 d                                         5 % (prestretched)                                                                          1 d  1 d  1 d  1 d  1 d                                Prestretched                                                                           10 % "        1 d  1 d  1 d  1 d  1 d                                windability                                                                            15 % "        1 d  1 d  1 d  2 d  2 d                                (good diameter)                                                                        20 % "        1 d  1 d  1 d  3 d  3 d                                         Repeated abrasion                                                    Abrasion 600 g load (cycles)                                                                         330  320  245  180  135                                resistance                                                                             Unilateral scrapes(g)                                                                       2753 2730 2635 2480 2510                               Heat Shock                                                                             200° C × 2 hrs                                                                 1 d  1 d  1 d  1 d  1 d                                resistance                                                                             240° C × 2 hrs                                                                 1 d  1 d  1 d  1 d  1 d                                (good diameter)                                                                        260°  C × 2 hrs                                                                1 d  1 d  1 d  2 d  2 d                                Breakdown                                                                              Normal state (KV)                                                                           11.3 12.0 12.5 13.4 12.1                               voltage  260° C × 168 hrs (KV)                                                          10.5 11.0 12.0 11.8 10.3                               __________________________________________________________________________

    __________________________________________________________________________    Crazing   In water  good                                                                              good                                                                              good                                                                              good                                                                              good                                      resistance                                                                    Cut through                                                                   temperature                                                                             2.1 Kg load (° C)                                                                472 470 458 421 418                                       Chemical  NaOH 3%   7 H 7 H 7 H 7 H 7 H                                       property                                                                      20° C × 24 hrs                                                             "5% "     7 H 7 H 7 H 6 H 6 H                                       (pencil hard-                                                                 ness)     "10% "    5 H 5 H 6 H 6 H 6 H                                                        0%                                                                       120° C ×                                                         30 min                                                                             5%                 Δ                                   Blister                                                                           R-22-Oil                                                                  resist-                                                                           (1:1)        10%        Δ                                                                           Δ                                                                           Δ                                   ance                                                                          test                                                                              70° C × 168 hrs                                                               0%                                                                       160° C ×                                                         30 min                                                                             5%                                                                            10%                Δ                                   __________________________________________________________________________

(Note 1) The mark (d) in the table shows a wire diameter. In theprestretched windability and the heat shock resistance tests in Table 1,1 d means that the test wire meets the requirement of the test using thesame diameter as that of the test wire and 2 d means that the test wiremeets the requirement in the same test using a diameter of 2 x (wirediameter).

(Note 2) The blister resistance test in the table was conducted bystretching the sample wire heat-treated for 15 minutes at 150° C to 0%,5%, or 10%, treating the sample in R-22-oil (1:1) for 168 hours at 70° Cby using an autoclave, treating the sample in a dryer for 30 minutes at120° C and for 30 minutes at 160° C, and then observing the state of theformation of blisters of the sample.

In addition, the mark in the table stands for the case where no foamswere observed; the mark stands for the case where foams observed hardly,and the mark Δ stands for the case where foams were observed by thenaked eye.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A solution for forming thermal resisting polymerscomprising a mixture of a prepolymer and blocked polyisocyanate compoundblocked with a compound of the formula ROH wherein R represents amonovalent aromatic group or a monovalent aliphatic group inapproximately stoichiometric equivalent amount to said prepolymer, saidmixture being dissolved in an organic acidic solvent, said prepolymerhaving been prepared by reacting an organic diisocyanate or diamine anda molar excess of 1, 2, 3, 4-butane-tetracarboxylic acid or an anhydridethereof and the prepolymer having an imide group in the main chain ofthe molecule and an acid group at the terminals of said molecule, saidblocked polyisocyanate being a blocked compound of an imidegroup-containing polyisocyanate prepared by reacting 1, 2, 3,4-butane-tetracarboxylic acid or an anhydride thereof with a molarexcess amount of an organic diisocyanate or said blocked polyisocyanatebeing a blocked compound of an amide or imide group-containingpolyisocyanate having isocyanate groups at both terminals of themolecule prepared by reacting the organic tribasic acid anhydriderepresented by the general formula ##STR2## wherein R₁ represents atrivalent organic group with a molar excess amount of an organicdiisocyanate.
 2. The solution for forming thermal resisting polymers asclaimed in claim 1 where either the prepolymer or the blockedpolyisocyanate compound is present in an amount up to 20% in excess ofits stoichiometric equivalent amount.
 3. The solution for formingthermal resisting polymers as claimed in claim 2 where the solution hasa non-volatile content of 20 to 60% by weight and a viscosity of 500 to150,000 c.p. (at 30° C).
 4. The solution for forming thermal resistingpolymers as claimed in claim 1 where said prepolymer is formed byreacting an organic diisocyanate or diamine and a molar excess of1,2,3,4-butanetetracarboxylic acid or an anhydride thereof at atemperature in excess of 30° C.
 5. The solution for forming thermalresisting polymers as claimed in claim 4 where said prepolymer is foundby reacting an organic diisocyanate or diamine and a molar excess of1,2,3,4-butanetetracarboxylic acid or an anhydride thereof at atemperature of 130° - 200° C.
 6. The solution for forming thermalresisting polymers as claimed in claim 4 where the prepolymer is formedby reacting an organic diisocyanate or diamine and a molar excess of1,2,3,4-butanetetracarboxylic acid or an anhydride thereof in an organicacidic solvent or an organic polar solvent.
 7. The solution for formingthermal resisting polymers as claimed in claim 6 where the totalconcentration in the prepolymer reaction system is 30 to 95% reactants.8. The solution for forming thermal resisting polymers as claimed inclaim 4 where the prepolymer is formed by reacting an organicdiisocyanate or diamine and a molar excess of1,2,3,4-butanetetracarboxylic acid or an anhydride thereof, the1,2,3,4-butanecarboxylic acid being present in an amount of 1.1 to 2.2moles per mole of organic diisocyanate or diamine.
 9. The solution forforming thermal resisting polymers as claimed in claim 8 where thediamine is reacted to form the prepolymer by a dehydration reaction atleast in part at 190° to 200° C for 2-3 hours, water being removed fromthe reaction system during reaction.
 10. The solution for formingthermal resisting polymers as claimed in claim 8 where organicdiisocyanate is reacted to form the prepolymer by a dehydration/carbondioxide removal reaction at least in part at 190° to 200° C for 2-3hours, water and carbon dioxide being removed from the reaction systemduring reaction.
 11. The solution for forming thermal resisting polymersas claimed in claim 10 where reaction is in an organic acidic solvent.12. A process for preparing a solution as claimed in claim 1 whichcomprises blending said prepolymer solution and said blockedpolyisocyanate in approximately stoichiometric equivalent amounts. 13.The solution for forming thermal resisting polymer as claimed in claim9, wherein reaction is carried out in an organic acidic solvent.
 14. Thesolution for forming thermal resisting polymer as claimed in claim 1,wherein said ROH is a cresol, a xylenol, a phenol or an alcohol.
 15. Thesolution for forming thermal resisting polymer as claimed in claim 1wherein said organic acidic solvent is cresol, xylenol, phenol or amixture thereof.